1. Field of the Invention
The present invention relates to a method for producing an optical element having a three-dimensional structural part that has a refractory index periodic structure equal to or less than a wavelength of visible light and also relates to the optical element obtained produced by the method.
For example, the invention relates to a method for producing an optical element having a three-dimensional structural part, such as a polarizing beam splitter, a phase plate, and a band-pass filter that has a structure with an in-plane period shorter than a wavelength of visible light on the surface, and to the optical element.
2. Description of the Related Art
Optical elements including a three-dimensional periodic structural part having a refractive index periodic structure equal to or less than a wavelength of visible light have been actively developed.
For example, USP No. 2007/0201135 suggests a polarizing element composed of a structure in which three layers of line-and-space structures overlap with a relative rotation of the line direction through 90°. In this case, a method using a titanium coupling agent as an adhesive is described as a method for bonding the structural part and the substrate. With this method, the titanium coupling agent is diluted by a factor of 60 with isopropyl alcohol, and the diluted agent is coated on the substrate by a spin coating method and pre-cured for 2 min at a temperature of 180° C., thereby forming an adhesive agent layer. The structural part and the substrate are then tightly joined by the adhesive agent layer, and bonding is performed by allowing the configuration to stay for 5 min at a temperature of 200° C. on a hot plate, while applying a load of 2 kg.
Japanese Patent Laid-Open No. 2001-74955 describes a method for joining structural parts by stacking substrates each having a structural part so that the structural parts come into contact and then heating in the H2 atmosphere at a temperature of 650° C. to fuse the structural parts. Where such method is used, covalent bonds or metal bonds can be realized by diffusion of hydrogen, and effective joining can be performed with metals or semiconductors.
However, the following problems are encountered when a bonding-joining method based on the above-described conventional examples is applied to a method for producing an optical element including a three-dimensional structural part.
Thus, in a case where the conventional procedure described in USP No. 2007/0201135 is applied to a method for producing a optical element that has a three-dimensional structural part with a period equal to or less than a wavelength of visible light, the method including a process of joining oxide materials, the adhesive layer penetrates into concave portions of the structural part under the effect of capillary forces.
The resultant problem is that the desired optical characteristics cannot be obtained.
Furthermore, in a case where the conventional procedure described in Japanese Patent Laid-Open No. 2001-74955 is applied to a method for producing a optical element that has a three-dimensional structural part with a period equal to or less than a wavelength of visible light, the method including a process of joining oxide materials, oxides become the object of joining.
The resultant problem is that covalent bonds or metal bonds produced by diffusion of hydrogen are difficult to realize and a sufficient joining strength cannot be obtained.
With the foregoing in view, it is an object of the invention to provide an optical element having a three-dimensional structural part that can resolve the problems associated with deterioration of optical characteristics that is caused by variations in the element shape in the conventional process and poor endurance caused by insufficient joining strength, and also to provide a method for producing the optical element.